The Aquarium Pharmaceuticals (API) Calcium Test Kit. Calcium is one of the most important elements in a reef aquarium. Reef invertebrates such as corals, crustaceans, mollusks and even some types of encrusting algae need calcium to grow and remain healthy. Over time, calcium levels drop as invertebrates use calcium in aquarium water. This kit allows you to easily and accurately measure calcium levels and maintain a healthy reef aquarium.